1. Field of the invention
The present invention relates to an analog-to-digital converter, and more particularly to an improvement in an analog-to-digital converter of the successive-approximation type having a string of resistors as a voltage divider.
2. Description of the Background
Prior analog-to-digital converters of the successive-approximation type successively compare an analog input voltage with a divided voltage from a string resistor circuit, and thereby convert the analog input voltage into a digital form. However, it is recognized that a high resolving power of the analog-to-digital converter causes a great increase in the number and resistance values of the resistors of the string resistor circuit, and further causes an increase in the length of a settling time necessary for analog-to-digital conversion. This results in a lowering of the speed of the analog-to-digital converter of this type. Furthermore, it is, in general, known that the string resistor circuit is fabricated by a plurality of diffusional or polysilicone resistors in the form of a semiconductor integrated circuit. This causes an increase in a resistance value of each of the resistors.